When glass or silica (hereafter all referred to as glass) optical fibers are to be used in a mode in which they will be strongly compressed or stretched longitudinally, as a non-exclusive example as a fiber optic strain sensor, the high Young's modulus of the glass and high shear stress, combined with its fragile nature, make it difficult to hold or ‘fix’ the fiber without slippage (‘creep’) or breakage, especially in a physically compact manner such as would be demanded of a sensor. Such creep causes a drift in, as a non-exclusive example, the ‘zero’ reading of a strain sensor based on the well-known fiber Bragg grating (FBG).
Adhesives such as epoxies are frequently used to fasten fibers, but the fiber, if stretched or compressed enough, can either creep through as much as several centimeters of epoxy or the fiber will move with respect to its buffer coating in such a way as to reduce the shear stress between the fiber and its buffer coating, or even separate from its buffer coating, causing a false and/or unstable optical signal.
Said buffer coating is usually applied during the fiber drawing process to protect the otherwise fragile glass. This buffer coating can be a polymer, carbon, metal or any of many materials or combinations of materials that suit the purpose of the user. A glass fiber that has never had its original buffer coating removed or damaged can be termed ‘pristine’. In ‘pristine’ condition, the glass fiber is as strong under stress as can be obtained.
If, as a non-exclusive example, an FBG is entirely or partially coated with adhesive when bonded to another body, a temperature or stress gradient along the adhesive bond can cause the FBG reflection peak to break up into multiple peaks, or ‘chirp’, thus interfering with an optical signal being unambiguously obtained from the sensor.
Several schemes have been devised in the prior art to overcome this deficiency by providing ‘creep free’ or ‘low creep’ fixing points of a compact length. These include, as non-exclusive examples, creating a lump in the glass by heating the glass fiber to its melting point and moving the two solid parts of the fiber toward each other to form an increase in the diameter of the fiber (a lump in the fiber) with the aim of using the lump to catch and hold the fiber; metalizing and soldering the glass; and a method using the compressive effect of a metal solder in a metal sleeve or other metal structure, to seal the fiber with an organic or inorganic buffer layer interposed between the glass fiber and the metal. The latter method keeps the fiber and its buffer in compression at temperatures below the solder melting point because of the higher expansion coefficient of the metal compared to the glass, maintaining a compression seal cushioned by the buffer material. However, the fiber can still shear away from its buffer coating under the metal. Many high temperature adhesives, such as those marketed by Cotronics, Inc., have been formulated to bind directly to glass. However, these require stripping the buffer layer and the fiber becomes fragile and prone to breakage when bent or otherwise stressed near the bond.
Optical fiber sensors and other devices are often in direct competition with electronic sensors that measure the same measurand. Optical sensors performing the same function as an electrical equivalent are usually the more expensive and may have a much larger, or at least much different, form factor than those of the electronic counterpart, thus putting them at a disadvantage. The counteracting advantages of optical sensors are, as non-exclusive examples, that they are immune to electromagnetic interference, explosion-proof, and can be made of all-dielectric materials that make them compatible with high voltages. These advantages, however, can frequently only be utilized by the user if the optical sensor fits in the space allowed with a similar form factor to those of existing electronic sensors and does not drift with time due to its internal construction.
A description of some of the mechanical and optical properties of glass fibers may be useful. Optical fibers that are bent with small radii (<1 cm) have two liabilities. One is the tensile stress that is developed in the outside edge of the fiber in the plane of the circle. Assuming a ‘pristine’ fiber with proper buffer layer and lack of damaging abrasion and contaminants present, the bend radius versus the time to fracture for various glass formulations, diameters and buffer coatings is well known, allowing for conditions that include the permeation of moisture and other damaging contaminates. Assuming a 125 micrometer glass diameter fiber in pristine condition with a conventional acrylate or polyimide buffer coating, an illustrative example would be one of bending it in about a 4 mm radius in a low humidity environment, which will produce a time to fracture of about 50 years, while the same fiber bent in about a 1.5 mm radius will fracture within minutes or hours. The other liability of bending fibers is that severe light intensity loss can be suffered, depending on the numerical aperture of the fiber. Several non-exclusive examples of acceptable bend radii for a 360° bend with respect to light loss per 360° loop are: A fiber with a numerical aperture of about 0.35 that can be bent in a radius of about 1 mm with light loss of only a few tenths dB, on the condition that the bend has to be fabricated under enough heating to relieve the stresses; a fiber with a numerical aperture of about 0.16 to 0.2 (known as a bend insensitive fiber) that can be mechanically bent and held in a radius of 3 mm with losses of <0.5 dB; a fiber with a numerical aperture of about 0.09 to 0.11 (common communications fiber, e.g., Corning SMF-28) that will lose significant light with a bend radius as large as 15 mm. Such light losses are illustrated in the data of FIG. 1.
We have surprisingly found that a compact low creep optical fiber fixing or anchoring method can be achieved by trapping the fiber in at least one bend, but preferably a series of bends of 180 degrees or more, in a medium or substrate that has mechanical properties (e.g., stiffness and hardness) to resist or prevent said fiber from cutting into it or distorting it when the fiber is subjected to stress, even over a long period of years. This trapping can be accomplished, as non-exclusive examples, by casting or molding the bent fiber into a substrate or body, adhesively bonding or soldering the optical fiber into a confining curved groove in a body or substrate, or by adhesively bonding, soldering or brazing the fiber into a confining tube, followed by winding of the tube at least at one end of the FBG-containing length into a compact form factor of the type generally described here. Further, the friction between the optical fiber and the at least one confining bend can be enhanced if said bend or bends contain at least one point of inflection in which the curvature of the optical fiber changes direction (i.e., the second derivative of a mathematical description of the path of the fiber passes through zero at least once). These non-exclusive examples of structures accomplishing low creep confinement or fixing of a section of optical fiber will be termed herein “fiber anchors” or “anchors”, and will necessarily provide for a means of fixing the anchor structure to at least one other body. This novel fixing means allows the use of inexpensive adhesives and has been demonstrated to hold FBG sensors in a stable “zero” condition over a period of several years.
Illustrative technology herein provides a method of fixing or binding optical fibers in an assembly compact enough to be used conveniently as an anchor or as an enabling part of a strain or temperature sensor while retaining low optical losses and the original buffer coating to prevent the fiber from being exposed to abrasion and other influences that could lead to breakage. By low optical losses is meant an insertion loss of less than 1 dB and preferably less than 0.3 dB. A novel method is disclosed herein that traps the fiber in a high friction environment that accomplishes long term creep resistance and yet provides breakage protection while maintaining a compact form factor that is simple and inexpensive. As a non-exclusive example, by compact form factor for a single point strain sensor is meant less than 100 mm in any dimension and preferably less than 75×25 mm in the plane of mounting of the strain sensor in order to enhance the placement of the sensor to measure strain at a specific point. This method is of particular benefit with fibers stressed in the longitudinal direction of the fiber, but is also of utility in cases of radial stress or combined axial and radial stresses. The novel and beneficial nature of the technology herein will become evident in the following description.
Further, if two or more of the herein-defined anchors are utilized on either side of an optical sensing element fabricated within the optical fiber, a sensor can be fabricated for the reliable measurement of such strain or vibration that is desired to be determined in a body to which both anchors can be firmly attached. In one non-exclusive example, strain can be used to transduce various parameters such as pressure and vibration to wavelength changes. Similarly, the movement between two different bodies can be measured by attaching one anchor to each of said bodies with an arbitrarily long length of taut optical fiber between them, said fiber containing at least one FBG. For the purposes of this disclosure, “an Optical Sensing Element” is defined to include at least one of a fiber Bragg grating (FBG), a Fabry-Perot etalon, a Mach-Zehnder interferometer or other interferometric optical device.